Antennas for use in a mobile radio communication terminal such as a cellular phone terminal are disclosed in Japanese Unexamined Patent Application Publication No. 2009-33742 (Patent Document 1), Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2007-524310 (Patent Document 2), Japanese Unexamined Patent Application Publication No. 2006-67259 (Patent Document 3), and Japanese Unexamined Patent Application Publication No. H9-153734 (Patent Document 4).
The antenna in Patent Document 1 is a dual-feed multi-band antenna. FIG. 1 shows a configuration of an antenna device in Patent Document 1. A first antenna element 11 is fed at a first feed point 13 provided on a substrate 1, and is grounded so as to be short-circuited to a ground circuit of the substrate 1 at a first short-circuit portion 14. A second antenna element 12 is fed at a second feed point 15 provided on the substrate 1, and is grounded so as to be short-circuited to the ground circuit of the substrate 1 at a second short-circuit point 16. The first short-circuit point 14 and the second short-circuit point 16 are provided between the first feed point 13 and the second feed point 15.
The first antenna element (radiating element) 11 operates in a substantially λ/4 mode, and the second antenna element (radiating element) 12 operates in a substantially λ/2 mode. The radiating element of the substantially λ/2 mode has a folded shape and its ground point is located near its feed point.
The antennas in Patent Documents 2 and 3 are dual-feed multi-band antennas in which two radiating elements have a common ground point. The feeding manner for the both antennas is capacitance feeding.
The antenna in Patent Document 4 is a single-feed single-band antenna in which a ground point is located near a feed point. The feeding manner for the antenna is direct feeding.
Patent Document 1 describes that isolation is improved by locating the ground points of the two radiating elements between the feed points of the two radiating elements. However, when the antenna device is installed (mounted) on a circuit board, the total number of terminal electrodes is four (the two feed points and the two ground points), which leads to an increase of cost and a decrease of reliability. Further, Patent Document 1 does not describe antenna efficiency. However, in general, if an electrode pattern of a substantially λ/4 mode is formed so as to have a folded structure and a ground point is located near a feed point, the loop diameter becomes small, and the radiation resistance becomes low, resulting in deterioration of the antenna efficiency.
In Patent Documents 2 and 3, two radiating elements seem to operate in a substantially λ/4 mode due to the structure. Further, there is no description concerning an operation in a substantially λ/2 mode, and the effect caused by a combination with a substantially λ/2 mode is not described.
Further, if, in the structures of the antennas disclosed in Patent Documents 2 and 3, the feeding manner is changed into direct feeding as in Patent Document 4, it will be expected that sufficient isolation cannot be ensured between the two radiating elements.